Possessive Case: The Apostrophe

[aysaa]

Hi,

1)Three days' work.
Three day's work.
Three day work.

2)Three weeks' courses
Three week's courses
Three week courses.

Do they have the same meaning?
Thanks.

 

[Tdol]

1 I would use the first.
2 I would use three-week courses.

 

[aysaa]

Which one is OK to use?

A three gallon jar or a three gallon's jar or a three gallons' jar.

Thanks again.

 

[bhaisahab]

Which one is OK to use?

A three gallon jar or a three gallon's jar or a three gallons' jar.

Thanks again.

Only the first one is correct.

 

[Barb_D]

I would add that I would consider it correct only with a hyphen. A three-gallon jar.

 

[BrunaBC]

If I may, I'd like to add that in this case there is no possession. Three-gallon is giving a characteristic to the jar. It works as an adjective here. Such as:
Twenty-day vacation, three-day holiday, etc.